- redesign box-shadow generator with modern UI

- add theme toggle with light/dark mode support
- implement tabbed interface for controls, presets, and shadow list
- add multiple shadow management with preview and delete options
- create predefined shadow presets (subtle, neon, 3D, etc.)
- improve controls with real-time value displays and linked inputs
- add shape controls (square, rounded, circle) for preview element
- implement toast notifications for clipboard operations
- make layout responsive with improved mobile support
- add code preview with syntax highlighting and copy feature

Author: TobiasMue91